I was originally in support of spit not targeting buildings...but after having put some more games under my belt I don't think spit needs a nerf. If you make spit not effect buildings, UB becomes too squishy to do any kind of dmg to buildings in the early/mid game. Like Erebus' bite, spit may seem overpowered but it is counterable.
All of the other assassins have good anti building skills of some form. Reg has his mines/range, Rook steals health and smashes, torch can freeze and fireball (which is quite maintainable), and UB can spit. Spit may need a slight dmg reduction, but then it is a DoT and is easily countered by pots, shields, heal, etc. If you ever take it's full dmg, then you haven't properly prepared for your opponent. Bottom line, probably no nerf needed.
As an assassin, I think UB needs spit to really have the ability to push. By not targeting buildings you take that away to a large extent.
I still think Oak shouldn't target buildings in last stand however. He can still push plenty well without last stand.